1. Field of the Invention
The present invention relates to a recording sheet package for a thermal printer, especially for a direct thermal printer that directly heats a thermosensitive recording sheet to print an image thereon.
2. Background Arts
Thermal printers may be roughly classified into two types: direct thermal printing type and thermal transfer type. The thermal transfer type includes wax transfer type and sublimation type. For each type printer, a particular type of recording sheet is used. The wax transfer type printer melts or softens ink on ink film, and transfers it to the recording paper. The sublimation type printer sublimates or disperses dye of ink film onto the recording sheet. The recording sheet for the wax transfer type consists of paper coated with a smoothing layer. The recording sheet for the sublimation type consists of paper coated with polyester resin.
The thermosensitive recording sheet for the direct thermal printing is usually provided for a full-color printing. For example, JPA 61-213169 discloses a thermosensitive color recording sheet. The thermosensitive color recording sheet is constituted of a support layer and at least three coloring layers overlaid thereon which respectively develop cyan, magenta and yellow when heated up to different temperature ranges from each other. Thus, gradually increasing heat energies are applied to the thermosensitive recording sheet to develop three colors sequentially from the most thermosensitive coloring layer to the least thermosensitive coloring layer. Each coloring layer after developing color is optically fixed prior to the thermal coloring of the next coloring layer, so that the just colored layer may not develop color any more even while it is heated by the heat energies applied for the next coloring layer. For this optical fixation, ultraviolet rays of a predetermined wavelength range are applied to the thermosensitive recording sheet, to destroy the coloring ability of the colored layer.
Because of the photosensitivity to the ultraviolet rays, if the thermosensitive recording sheet is exposed to ambient light or light from a widely used fluorescent lamp or the like, for a certain time, the coloring ability is remarkably deteriorated. For this reason, the thermosensitive recording sheets must be preserved in a light-tight fashion. Moreover, since moisture has a great influence on printing quality in either type of recording sheet, it is desirable to protect the recording sheets from moisture as well as light even after they are loaded in the thermal printer, not to mention during their shipment and preservation.
To use the recording sheets, they are ordinarily loaded in a paper feeding cassette that is attached to a thermal printer. On loading the recording sheets in the paper feeding cassette, there have been risks of placing the recording sheets in a wrong posture, soiling the recording surface by the user's hand, or exposing the recording sheet to ambient light of an intolerable amount. Beside that, it has been uneasy to pile up the recording sheets neatly in the paper feeding cassette. If the recording sheets are loosely loaded, the recording sheets tend to get jammed in the printer.
To facilitate loading the recording sheets safely in a right posture, many types of recording sheet packages containing a pile of recording sheets in a casing have been suggested. JPA 5-116774 discloses a recording sheet package, wherein a portion of the casing is cut off along a line of cutting perforations to provide a paper feed-out opening for the recording sheet, and thereafter the package is loaded in a paper feeding cassette. The recording sheets are protected from light and moisture while being contained in the casing, and the users need not touch the recording sheets to load the recording sheets. However, the need for cutting the casing along the perforations makes this recording sheet package inconvenient.
Furthermore, in the recording sheet package of this prior art, an opening is formed through a bottom wall of the casing in connection to the paper feed-out opening concurrently with the paper feed-out opening being formed by cutting off the predetermined portion of the package casing. This bottom opening permits a push-up member to push up the recording sheets and presses the topmost recording sheet of the pile onto feed rollers that are inserted into the casing from upside of the paper feed-out opening. Therefore, this recording sheet package cannot sufficiently protect the recording sheets from light, moisture and dusts when it is unloaded from the paper feeding cassette. However, if the pile of recording sheets is not pressed onto these feed rollers by such a push-up member, the feed roller could not feed out the recording sheet when the remainder of recording sheets in the package reduces to a certain amount. Also, the piled recording sheets would be loosened, so the light-tightness and the moisture-tightness would be lowered as the remaining number of recording sheets decreases.